1. Field of the Invention
The present invention is related to the field of telecommunications, and in particular, to single ended line probing (SELP) in a Digital Subscriber Line (DSL) system using a transformerless hybrid.
2. Background
DSL communications use copper telephone lines (e.g., twisted pair) for high-speed data transmission. A major problem for DSL service providers is to accurately qualify a subscriber""s local loop (sometimes referred to as xe2x80x9cprobing the linexe2x80x9d) prior to the deployment of DSL service. In general, line probing involves measuring line parameters such as loop capacitance and loop resistance. A typical approach for probing the line requires a first handset to be attached to the telephone line at the telephone company""s Central Office (CO) location and a second handset to be attached to the telephone line at the customer premises equipment (CPE) location. Thus, human interaction is required at two points of the telephone line, including a service call to the CPE location, which increases the cost of deployment. SELP techniques eliminate the need for a service call to the CPE location and the additional costs of such service.
Conventional SELP techniques use a resistor-capacitor (RC) circuit model to estimate the length of a transmission medium, as shown in FIG. 1A. The RC circuit model typically includes the known source resistance Rs of the line and the unknown line capacitance C1. The line capacitance is approximately proportional to the length L of the transmission medium. A Direct Current (DC) pulse is applied to the line and the charge-up time tc is monitored. This function is sometimes provided on a digital multimeter. Once the charge-up time is known (and given a value for Rs), the line capacitance C1 can be estimated along with the line length. FIG. 1B is a graph illustrating the voltage across the line capacitance C1 as a function of time.
Unfortunately, the probing technique described above cannot be employed in a typical CO DSL modem. This is because the resistance of the RC circuit model typically includes the source resistance Rs, but ignores the line resistance. This is a reasonable approximation of the resistance when the source resistance is much larger than the line resistance. In conventional DSL modem boards, however, the output resistance of the source is typically not significantly larger than the line resistance. As such, ignoring the line resistance degrades the accuracy of the loop length estimate. Moreover, such conventional line probing techniques fail to consider the characteristics of a DSL modem having a transformerless hybrid.
Accordingly, there is a need for a new SELP technique that is suitable for use with DSL modems having transformerless hybrid circuits, and that can provide an accurate estimate of subscriber loop length without requiring a service call to the CPE location.
The present invention is directed to SELP techniques for devices (e.g., DSL modem) having transformerless hybrid circuits. These SELP techniques provide an accurate estimate of the length of a transmission medium (e.g., subscriber loop in a DSL system) by computing a transfer function that accounts for the characteristics of the transformerless hybrid circuit and the transmission medium. The transfer function is then related to the length of the transmission medium at one or more frequencies.
One embodiment of the present invention provides a method for estimating the length of a transmission medium included in a communication system having a transformerless hybrid circuit. The method includes transmitting a probe signal over the transmission medium by way of the transformerless hybrid circuit, and measuring a reflected version of the probe signal received from the transmission medium. The method proceeds with determining a transfer function characterizing the transmission medium and the transformerless hybrid circuit based on the transmitted probe signal and the reflected version of the probe signal, and estimating the length of the transmission medium based on the transfer function.
Another embodiment of the present invention provides a system for estimating the length of a transmission medium included in a communication system having a transformerless hybrid circuit. The system includes a transformerless hybrid circuit for transmitting a probe signal over the transmission medium. A processor is operatively coupled to the transformerless hybrid circuit (or included therein) for determining a transfer function characterizing the transmission medium and the transformerless hybrid circuit based on the transmitted probe signal and a reflected version of the probe signal received from the transmission medium. The processor estimates the length of the transmission medium based on the transfer function.